feat(playback): stream buffering UI, M4A moov-at-end streaming, hot-cache spill (#737)

* feat(playback): stream buffering UI, ranged M4A tail prefetch, demuxer fix

Defer seekbar/progress until HTTP stream is armed for both legacy and
RangedHttpSource; show buffering overlay on cover art. Add MP4 tail
prefetch and Symphonia isomp4 bounded-mdat/moov-at-EOF probing so
moov-at-end M4A can start without reading the full mdat.

* feat(hot-cache): spill large ranged streams to disk for promote

When a ranged HTTP download completes above the 64 MiB RAM promote cap,
write the existing buffer once to app-data stream-spill/ and register it
for hot-cache promote (rename) and replay via fetch_data. Analysis seeds
from the spill file up to the local-file cap (512 MiB).

* fix(ui): stream buffering — grayscale cover and static clock icon

Desaturate player and queue cover art while isPlaybackBuffering; keep a
non-animated clock overlay for visibility without the spinning animation.

* fix(playback): review follow-up — tests, i18n, spill cleanup, changelog

Clippy and test layout fixes; stream spill orphan cleanup on startup;
buffering flag guard in progress handler; bufferingStream in all player
locales; CHANGELOG and contributor credits for stream/M4A work.

* docs: attribute stream buffering and M4A streaming to PR #737

* test(audio): avoid create_engine in stream spill unit test

CI runners have no audio output device; test spill take/consume via
the Mutex slot only, matching install_stream_completed_spill tests.

src/store/playbackProgress.test.ts

@@ -19,12 +19,14 @@ afterEach(() => {
 describe('getPlaybackProgressSnapshot', () => {
   it('starts at zero', () => {
     const snap = getPlaybackProgressSnapshot();
-    expect(snap).toEqual({ currentTime: 0, progress: 0, buffered: 0 });
+    expect(snap).toEqual({ currentTime: 0, progress: 0, buffered: 0, buffering: false });
   });
 
   it('returns the latest snapshot after an emit', () => {
     emitPlaybackProgress({ currentTime: 42, progress: 0.5, buffered: 0.7 });
-    expect(getPlaybackProgressSnapshot()).toEqual({ currentTime: 42, progress: 0.5, buffered: 0.7 });
+    expect(getPlaybackProgressSnapshot()).toEqual({
+      currentTime: 42, progress: 0.5, buffered: 0.7, buffering: false,
+    });
   });
 });
 
@@ -34,8 +36,10 @@ describe('subscribePlaybackProgress', () => {
     subscribePlaybackProgress(cb);
     emitPlaybackProgress({ currentTime: 1, progress: 0.1, buffered: 0.2 });
     expect(cb).toHaveBeenCalledTimes(1);
-    expect(cb.mock.calls[0][0]).toEqual({ currentTime: 1, progress: 0.1, buffered: 0.2 });
-    expect(cb.mock.calls[0][1]).toEqual({ currentTime: 0, progress: 0, buffered: 0 });
+    expect(cb.mock.calls[0][0]).toEqual({
+      currentTime: 1, progress: 0.1, buffered: 0.2, buffering: false,
+    });
+    expect(cb.mock.calls[0][1]).toEqual({ currentTime: 0, progress: 0, buffered: 0, buffering: false });
   });
 
   it('returns an unsubscribe that detaches the listener', () => {